The field of the present invention is electronic test instruments. More particularly, the present invention relates to devices for measuring a cable length using a time domain reflectometry system.
When installing or maintaining a cable system, it is often desirable to measure the length of a cable. For example, an installer may desire to know how many feet of cable are left in a spool of wire. By knowing the length of cable on the spool, the operator may select a length of cable to install that does not have unnecessary splices. In another example, a repairperson may desire to locate a physical fault in a cable. Often times cables are buried underground or hidden within walls, so it is highly desirable to locate the physical disruption in the cable so that demolition and construction damage may be minimized. Further, by accurately locating the fault, the fault may more quickly be found and communication or other service be returned more promptly. It will be appreciated that a fault may be a break, a short, or other disruption to a cable system.
Cable length is often determined using a device called a time domain reflectometer (TDR). A time domain reflectometer is typically configured as a portable hand held device which may be conveniently coupled to one end of a cable. An operator configures the TDR to the parameters of the cable type being measured, and couples the TDR to the cable. Once connected, the time domain reflectometer initiates a measurement cycle by sending a signal pulse onto the cable. When the pulse reaches the end of the cable or a disruption on the cable, the pulse bounces off the end or disruption and is received back at the TDR. The time duration that it took the pulse to travel from the TDR to the end of the cable and from the end of the cable back to the TDR is indicative of the cable length.
A signal traveling on a cable travels at a speed that is a factor of the speed of light in a vacuum. This factor may be different for different cables, and is called the NVP (velocity of propagation). The NVP is typically about 0.7, but depending on cable type they vary between about 0.4 and about 0.9. In this regard the TDR may be calibrated for a particular NVP, and the round trip duration time for the pulse may be used to indicate the cable length.
Many TDRs are analog devices which display a cable length on an analog meter. These analog TDRs give a general indication of cable length, but typically do not have accuracy sufficient to narrowly identify a disruption in the cable or accurately present a cable length. Further, the analog devices are subject to variations due to temperatures and environment, and are subject to degraded performance over time.
It would be desirable to have a digital TDR that could avoid many of the problems present in an analog device. However, known digital TDRs do not provide sufficient accuracy, and typically are only accurate to within about 3 to 4 feet. Although this is more accurate than the typical analog device, it would still be highly desirable to locate disruptions and cable lengths more narrowly. For example, it may be highly desirable to have a digital TDR that could find a disruption to within less than one-foot resolution. In designing such a digital tool, it would be useful for the hypothetical digital TDR to operate on NVPs ranging from about 0.40 to about 0.90, and be able to measure lengths ranging from less than one foot to greater than 3,000 feet with a less than 1-foot resolution. At one extreme, when measuring a 3,000 foot cable at 0.4 NVP with such a hypothetical TDR, the time delay would be in excess of 15 microseconds. At the other extreme when at 1 foot and 0.99 NVP, the time delay would be around 2 nanoseconds. Accordingly, a hypothetical TDR designed to meet these specifications would need to measure duration periods ranging about 2 nanoseconds to greater than 15 microseconds with a time resolution of less than 50 picoseconds. Converting these requirements into a component structure would mean that such a hypothetical device may require a up to 19 bit counter operating at about 20 gigahertz. A digital TDR incorporating such a device, even if available, would be too expensive and draw too much power for current applications.
TDRs also suffer from a problem known as the dead zone problem. The dead zone problem relates to the shortest time delay that the TDR can measure. Typical dead zones for known TDRs are in the range of 10–20 nanoseconds which corresponds to about 3 to 6 feet of cable. If the cable has a disruption in this dead zone then the TDR will not report the disruption, or may report the disruption as being in a different location. It would be highly desirable to have a TDR that could report all the disruptions on the cable including those within a few feet of the TDR.
Accordingly there exists a need for a digital TDR that is accurate, able to measure short cable lengths and is easy and economical to build.